Due to request for low abrasion on rotational elements to achieve an extended life and for low extent of noise, fluid dynamic bearings (FDB) have been used in conventional fan motors and hard disk drive motors.
In a typical FDB, a shaft is pivotably inserted into a bearing sleeve with a bearing clearance formed therebetween. Pressure-generating grooves are formed on either the outer peripheral surface of the shaft or the inner peripheral surface of the bearing sleeve. The bearing clearance is filled with lubricant oil that provides a medium through which a dynamic fluid pressure field is generated upon relative rotation between the bearing sleeve and the shaft. During normal operation, the spinning of the shaft sets up a steady pressure field around the bearing clearance that separates the shaft and the bearing sleeve and thus prevents metal-to-metal contact.
FIG. 4 shows a dynamic pressure-generating groove pattern of a so called “herringbone” type. Each groove is V-shaped and has two branches 87a, 87b having a common intercrossing area 88. Suppose a top side of the groove pattern faces outside, so the lubricant oil in the groove at the top side exposes to atmosphere. Sealing measures must be taken to prevent leakage of the lubricant oil at the top side. When the shaft rotates, the lubricating oil is driven from ends of the branches 87a, 87b to the intercrossing area 88 to generate a high pressure. At the same time, because a part of the lubricating oil is moved to the intercrossing area 88, the lubricating oil remaining near the ends of the branches 87a, 87b generates a very low pressure. This low pressure is required to be even lower than the outside atmosphere pressure so that the lubricant oil at the top edge does not flow outside. However, if the lower pressure generated by the lubricating oil at the top edge is very close to the outside atmosphere pressure, the lubricating oil is still possible to leakage, when the motor is subject to vibration during use or the motor is used in a location where the outside atmosphere pressure is lowered. Therefore, the lower pressure generated by the lubricating oil at the top edge is desired to be low enough.
For the foregoing reasons, there is a need for a fluid bearing having an improved capability to prevent leakage of lubricating oil.